Second order neuron
axon of sensory neuron synapses on interneuron in CNS; from spinal cord/brainstem to thalamus
Third order neuron
if sensation is to reach our "awareness," the second-order neuron synapses on a third-order neuron in the thalamus and thalamus relays info to the cerebral cortex for "perception"

allow eyes to follow a moving object while maintaining image on fovea centralis

visual acuity and astigmatism

light rays → bend (refraction) as they pass through convex cornea & lens
• passing from air to higher density structures (pass through slower)
• cornea is fixed; shape of lens is adjustable

concave and convex

concave

surfaces diverge light waves

convex

surfaces converge light waves to a focal point

emmetropia

• normal eye has good visual acuity (sharpness of vision)
• incoming light is sharply focused on fovea centralist of retina
• lens inverts image on retina
∙ left side of visual field → projected only right side of retina
∙ upper portion of field → projected downward

myopia

(nearsightedness) eyes are too long, light rays focus in front of retina
• individuals can see near; not far
• corrected with a concave lens
• tested for with Snellen chart

snellen eye chart

tests for myopia

normal vision - person can read to line marked 20/20
20/40 vision - individual must stand 20 ft from line that normal person can see at 40ft

hyperopia

(farsightedness) eye is too short light rays focus behind retina
• individuals can see far; not near
• corrected with a convex lens

astigmatism

visual defect produced by abnormal curvature of cornea or lens or irregularity on their surface
• vision is distorted & blurred
• corrected with uneven lens
• tested with an astigmatism chart

accommodation

ability of eye to focus on objects at different distances

lens is flattened for distant vision →

ciliary muscles are relaxed & suspensory ligaments are taut

no PNS stimulation

lens is more spherical for close-up vision →

ciliary muscles are contracted & suspensory ligaments are relaxed

PNS stimulation

presbyopia

("old eyes") ability to lose focus on close objects decrease with age
• decline in elasticity of lens
• tested by measuring the closest point at which an individual can focus clearly
• e.g. 10 year old near pt of vision ~ 8cm; 70 year old ~ 100cm